Water-absorbent agent, method for production thereof, and water- absorbent composition

ABSTRACT

A method for the production of a water-absorbent agent comprises mixing an absorbent resin containing a carboxyl group with an additive soluble in the aqueous solution of at least one member selected from the group consisting of inorganic acids, organic acids, and polyamino acids and a cross-linking agent capable of reacting with the carboxyl group. A water-absorbent agent obtained by adding from 0.005 to 8 parts by weight of an epoxy compound to 100 parts by weight of a water-absorbent resin, exhibiting an absorption capacity without load of at least 45 (g/g), an absorption capacity under load of 20 g/cm 2  of not less than 30 (ml/g) and having a residue amount of an epoxy compound of not more than 2 ppm. A water-absorbent composition comprises a polyamino acid (salt) and a water-absorbent resin containing a carboxyl group.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a water-absorbent agent, a method forthe production thereof, and a water-absorbent composition. Moreparticularly, it relates to a water-absorbent agent manifesting as highan absorption capacity even under load as without load, excelling insafety as evinced by the absence from the resin surface of a residue ofthe cross-linking agent of high reactivity, and befitting sanitarymaterials and to a method for the production thereof. It also relates toa novel water-absorbent composition which exhibits an excellentwater-absorbent property to such body liquid as urine, blood, andsecretions, enjoys unusual gel stability, and also excels in theflowability after absorbing moisture, the ability to resist staticcharge, and the rustproofing effect.

[0003] 2. Description of the Prior Art

[0004] In recent years, the water-absorbent resins have been inextensive use as one of the component materials of such sanitarymaterials as disposable diapers and sanitary napkins for the purpose ofabsorbing body liquid.

[0005] The water-absorbent resins of this class heretofore known to theart include partially neutralized cross-linked polyacrylic acids(JP-A-55-84,304, JP-A-55-108,407, JP-A-55-133,413, U.S. Pat. No.4,654,039, and U.S. Pat. No. 4,286,082), hydrolyzed starch-acrylonitrilegraft polymers (JP-A-46-43,995 and U.S. Pat. No. 3,661,815), neutralizedstarch-acrylic acid graft polymers (JP-A-51-125,468 and U.S. Pat. No.4,076,663), saponified vinyl acetate-acrylic ester copolymers(JP-A-52-14,689 and U.S. Pat. No. 4,124,748), hydrolyzed acrylonitrilecopolymers or acrylamide copolymers (JP-A-53-15,959, U.S. Pat. No.3,935,099 and U.S. Pat. No. 3,959,569), cross-linked derivativesthereof, cross-linked carboxymethyl cellulose (U.S. Pat. No. 4,650,716and U.S. Pat. No. 4,689,408, and cross-linked polymer of cationicmonomers (JP-A-58-154,709, JP-A-58-154,710, U.S. Pat. No. 4,906,717,U.S. Pat. No. 5,075,399, and EP0304143), cross-linked isobutylene-maleicanhydrous copolymers (U.S. Pat. No. 4,389,513), and cross-linkedcopolymers of 2-acrylamide-2-methylpropanesulfonic acid with acrylicacid (EP068189), for example.

[0006] The characteristic qualities which the water-absorbent resins aredesired to possess include high water-absorption capacity and fine speedof absorption to be manifested on contract with aqueous liquids, liquidpermeability, high strength exhibited by the gel swelled with liquid,and ability to aspirate water from the substrate impregnated withaqueous liquid, for example. These qualities are not necessarilycorrelated positively to one another. For example, such physicalproperties as the liquid permeability, the gel strength, and theabsorption speed are lowered in proportion as the water-absorptioncapacity is heightened.

[0007] As a measure to improve the various water-absorption propertiesof the water-absorbent resins in finely balanced levels, the techniqueof cross-linking the surface regions of water-absorbent resin particleshas been known. Various methods have been heretofore proposed concerningthe technique.

[0008] For example, methods using polyhydric alcohols (JP-A-58-180,233,JP-A-61-16,903, U.S. Pat. No. 4,734,478, and U.S. Pat. No. 5,164,459), amethod using polyglycidyl compounds, polyadiridine compounds, polyaminecompounds, and polyisocyanate compounds (JP-A-59-189,103), methods usingglyoxal (JP-A-52-117,393 and U.S. Pat. No. 4,051,086), methods usingpolyvalent metals (U.S. Pat. No. 4,043,952, JP-A-51-136,588,JP-A-61257,235, and JP-A-62-7,745), methods using a silane couplingagent (U.S. Pat. No. 4755560, JP-A-61-211,305, JP-A-61-252,212, andJPA-61-264,006), a method using a monoepoxy compound (JP-A-61098,121), amethod using an epoxy group-containing polymeric compound (U.S. Pat. No.4,758,617), a method using an epoxy compound and a hydroxy compound(JP-A-02-132,103), and a method using an alkylene carbonate (DE-4020780)severally as a cross-linking agent have been known. Besides, methodsrequiring the presence of an inactive inorganic powder (JP-A-60-163,956,JPA-60-255,814, and U.S. Pat. No. 4,587,308), a method requiring thepresence of a dihydric alcohol (JP-A-01-292,004), a method requiring thepresence of water and an ether compound (JP-A-02-153,903), and a methodthe presence of the alkylene oxide adduct of a monohydric alcohol, anorganic acid salt, lactam, etc. (EP-0555692) in the process of across-linking reaction have been known.

[0009] These methods indeed improve the balances of the various physicalproperties of the absorbent resins. The degrees of these improvements,however, hardly deserve to be rated fully satisfactory. Thus, thewater-absorbent resins have room for further improvement in qualities.Particularly, in recent years, the desirability of perfecting awater-absorbent resin which is capable of exhibiting outstandingwater-absorption properties under load, especially water-absorptioncapacity under load while keeping at a high level the water-absorptioncapacity without load, i.e. one of the basic physical properties of theconventional water-absorbent resins, has been finding popularrecognition. Of course, the water-absorption capacity without load andthe water-absorption capacity under load are generally in acontradicting relation. With all the methods heretofore known to theart, the needs of further technical improvements have not been fullysatisfied.

[0010] Besides, the water-absorbent resins have the problem of requiringuse of a cross-linking agent in cross-linking the surface region ofwater-absorbent resin particles and inevitably suffering thiscross-linking agent to persist as a residue on the surface of resinparticles. This problem is absent when the cross-linking agent so usedhas low reactivity and high safety as in the case of a polyhydricalcohol. When the cross-linking agent has high reactivity as in the caseof an epoxy compound, though the cross linking in the surface regionproceeds quickly and, the fact remains that the cross-linking agentitself has the nature of irritating the skin and, when suffered topersist on the surface of resin, jeopardizing the safety of sanitarymaterials using the cross-linked absorbent resin. This gives rise to anew problem from the viewpoint of safety.

[0011] The method which, for the purpose of decreasing the amount of thefraction of a cross-linking agent being used for cross-linking thesurface region which persists as a residue in the resin, comprisesinitiating the cross-linking reaction of the surface region of resin ata specific water content and adding a specific amount of water to thecross-linking reaction system while the reaction is in progress has beenknown (JP-A-03-295,705). This method, however, not only entails acomplicated process but also fails to decrease the amount of a residueof the cross-linking agent sufficiently.

[0012] The methods described above fulfill, though not thoroughly, theimprovement of the balances of various physical properties ofwater-absorbent resins and the enhancement of the absorption capacity ofthe water-absorbent resins under load. They at times prove undesirablefrom the viewpoint of safety because the cross-linking agent used forcross-linking the surface region of the water-absorbent resins particlespossibly leaves behind a persistent residue in the polymer.

[0013] They have further problems of low flowability (blocking) with thehigh hygroscopicity of the produced water-absorbent resins itself whichrenders difficult the work of fabricating the water-absorbent resinsunder the conditions of high temperature and high humidity, thesusceptibility of the water-absorbent resins to static charging whichdegrades the ease of handling, and the ability of the water-absorbentresins to gather rust on metal which induces corrosion of the devices tobe used for the work of fabrication. Further, when the liquid to beabsorbed is blood, the absorption capacity of the water-absorbent resinsis degraded because the blood components of the blood being absorbed bythe water-absorbent resins envelope the individual water-absorbentresins particles and eventually obstructing them from continuing theirfunction as a water-absorbent. The water-absorbent resins heretoforeknown have not been always given fully satisfactory results particularlyin such applications as sanitary napkins, for example.

[0014] Then a method which attains the purpose of improving thewater-absorbing power of water-absorbent resins relative to blood byadding such compounds as common salt and polyether to thewater-absorbent resins has been proposed (JP-A-58-501,107,JP-A-55-50,357, JP-A-54-70,694, US-Re 33839, EP-A-0009977, U.S. Pat. No.4,190,563, etc.). Indeed, this method is effective to some extent inpreventing the individual water-absorbent resins particles fromconglomeration, it actually cannot bring about the expected effectbecause the added compound either degrades the water-absorbent resins'own absorption capacity or fails to increase the suction power to blood.

[0015] An object of this invention, therefore, is to provide anabsorbent agent manifesting as high an absorption capacity even underload as without load, excelling in safety as evinced by the absence fromthe resin surface of a residue of the cross-linking agent of highreactivity, and befitting sanitary materials and to a method for theproduction thereof.

[0016] Another object of this invention is to provide a novelwater-absorbent composition, wherein a water-absorption capacity,especially an absorption property to blood is sufficient and workabilityand safety are satisfied, a method for the production thereof, and awater-absorbent structure and a water-absorbent article both containingthe water-absorbent composition.

[0017] The water-absorbent composition of this invention is adapted fordisposable diapers, sanitary napkins, jigs for use on patients ofincontinence, such water-absorbent articles as injury protectingmaterials and injury curing materials which serve the purpose ofabsorbing body liquid, or biotechnology applications.

SUMMARY OF THE INVENTION

[0018] The inventors, as a result of a diligent study pursued foraccomplishing the objects mentioned above, have found that when aspecific additive is used in the production of a water-absorbent agentby mixing a water-absorbent resin containing a carboxyl group with across-linking agent capable of reacting with the carboxyl group, thewater-absorbent agent exhibiting a preeminently excellentwater-absorption capacity under load while retaining various absorptionqualities, particularly water-absorption capacity without load, at highlevels and, even when the cross-linking agent has high reactivity,suppressing notably the amount of a residue of the cross-linking agentis easily obtained and that a specific water-absorbent composition isremarkably superior in absorption of blood. This invention has beenperfected on the basis of this knowledge.

[0019] The various objects described above are accomplished by a methodfor the production of a water-absorbent agent which comprises mixing awater-absorbent resin containing a carboxyl group with an additivesoluble in an aqueous solution of at least one member selected from thegroup consisting of inorganic acids, organic acids, and polyamino acidsand a cross-linking agent capable of reacting with the carboxyl group.

[0020] This invention also concerns the method which comprises mixingthe water-absorbent resin with the additive and the cross-linking agentand subsequently subjecting the mixture to a heat treatment at atemperature in the range of from 100° to 230° C. This invention alsoconcerns the method which comprises preparatorily mixing the additiveand the cross-linking agent and subsequently adding the mixture to thewater-absorbent resin. This invention also concerns the method, whereinthe additive is at least one member selected from the group consistingof saturated organic carboxylic acids, saturated inorganic acids ofelements of Group 3 in the periodic table of elements, andpoly(monoaminodicarboxylic acids). This invention also concerns themethod, wherein the absorbent resin containing a carboxyl group isobtained by polymerizing a hydrophilic monomer having acrylic acidand/or a salt thereof as a main component thereof. This invention alsoconcerns the method, wherein the cross-linking agent capable of reactingwith the carboxylic group is an epoxy compound. This invention alsoconcerns the method, wherein the acid dissociation index (pKa value) ofthe additive is in the range of from 2.0 to 4.0. This invention alsoconcerns the method, wherein the additive is a saturated organiccarboxylic acid. This invention also concerns the method, wherein thesaturated organic carboxylic acid is at least one compound selected fromthe group consisting of citric acid, succinic acid, and lactic acid.This invention also concerns the method, wherein the mixing is effectedby the use of from 0.01 to 5 parts by weight of water based on 100 partsby weight of the absorbent resin. This invention also relates to themethod, wherein the water-absorbent resin comprises irregular brokenparticles of an average particle diameter of from 200 to 600 μm andincludes not more than 10% by weight of a fraction of particles of lessthan 150 μm in diameter. This invention also concerns the method,wherein the additive is used at a ratio in the range of from 0.01 to 5parts by weight to 100 parts by weight of the water-absorbent resin.This invention also relates to the method, wherein the water-absorbentresin has a water content in the range of from 1 to 10%.

[0021] The objects mentioned above are further accomplished by awater-absorbent agent obtained by adding from 0.005 to 8 parts by weightof an epoxy compound to 100 parts by weight of a water-absorbent resincontaining carboxyl group, exhibiting an absorption capacity withoutload of not less than 45 (g/g) and an absorption capacity under a loadof 20 g/cm² of not less than 30 (ml/g) and having a residue amount of anepoxy compound of not more than 2 ppm.

[0022] This invention also concerns the water-absorbent agent, wherein ahydroxyl group-containing saturated organic acid is contained at aconcentration in the range of from 0.01 to 5% by weight (based on thewater-absorbent resin). This invention also concerns the water-absorbentagent, wherein the absorption capacity under load of 20 g/cm² is notless than 35 (ml/g) and the epoxy compound is no longer detected.

[0023] The objects mentioned above are further accomplished by awater-absorbent composition comprising a polyamino acid (salt thereof)and a water-absorbent resin containing a carboxyl group.

[0024] This invention also concerns the composition, wherein thewater-absorbent resin is a cross-linked polymer of acrylic acid (salt).This invention also concerns the composition, wherein the ratio of thepolyamino acid (salt) is from 0.01 to 30 parts by weight to 100 parts byweight of the water-absorbent resin. This invention also concerns thecomposition, wherein the polyamino acid (salt) is at least one memberselected from the group consisting of polyaspartic acid (salt),polyglutamic acid (salt), and polylysine (salt).

[0025] The various objects are further accomplished by a water-absorbentcomposition comprising a water-absorbent resin exhibiting an absorptioncapacity of at least 25 g/g to physiological saline solution withoutload and a suction power of not less than 14 g/g to physiological salinesolution and a suction power of at least 7 g/g to bovine blood.

[0026] The various objects are further accomplished by a water-absorbentstructure comprising 10 to 95 parts by weight of a water-absorbentcomposition containing a polyamino acid (salt) and a water-absorbentresin having a carboxyl group and 90 to 5 parts by weight of ahydrophilic fibers.

[0027] This invention also concerns the water-absorbent structure whichfurther comprises up to 40 parts by weight of a thermoplastic fibers.This invention also concerns the water-absorbent structure, wherein thedensity is in the range of from 0.06 to 0.5 g/cc.

[0028] The various objects are further accomplished by a water-absorbentstructure comprising 50 to 95 parts by weight of a water-absorbentcomposition formed of a water-absorbent resin exhibiting an absorptioncapacity of at least 25 g/g to physiological saline solution withoutload and a suction power of not less than 14 g/g to physiological salinesolution and a suction power of at least 7 g/g to bovine blood and 50 to5 parts by weight of a hydrophilic fibers.

[0029] This invention also concerns the water-absorbent structure, whichfurther comprises up to 40 parts by weight of thermoplastic fibers basedon 100 parts by weight of the hydrophilic fibers. This invention alsoconcerns the water-absorbent structure, wherein the density is in therange of from 0.06 to 0.5 g/cc.

[0030] The various objects are further accomplished by a water-absorbentarticle furnished with the water-absorbent structure mentioned aboveand/or the water-absorbent composition mentioned above.

[0031] The method of this invention permits easy and convenientmanufacture of a water-absorbent agent which exhibits as high anabsorption capacity even under load as without load, excels in safety,and befits sanitary materials.

[0032] The water-absorbent agent of this invention exhibits a highabsorption capacity without load and a very high absorption capacityunder load, allows no occurrence on the resin surface of a residue of across-linking agent of high reactivity, permits high diffusion ofliquid, and does not readily migrate in or exfoliate from pulp and,therefore, is advantageously adapted for such absorbent articles asdisposable diapers and sanitary napkins and other sanitary materials.

[0033] The operation and the effect of the polyamino acid (salt) in thewater-absorbent composition of this invention have not yet been fullyelucidated. It has no problem from the viewpoint of safety and hygiene.The polyamino acid (salt), when added in a small amount to thewater-absorbent resins, enables the quality of the water-absorbentresins to be conspicuously enhanced. The water-absorbent composition ofthis invention, therefore, is advantageously adapted for disposablediapers, sanitary napkins, jigs for use on patients of incontinence,such water-absorbent articles as injury protecting materials and injurycuring materials which serve the purpose of absorbing body liquid, orbiotechnology applications. Thus, this invention contributes immenselyto the growth of the industry concerned.

BRIEF DESCRIPTION OF THE DRAWING

[0034] The FIGURE is a cross section of a device used in this inventionfor the measurement of absorption capacity under load.

EXPLANATION OF PREFERRED EMBODIMENT

[0035] The carboxyl group-containing water-absorbent resin useful forthis invention is a well-known resin which is obtained, for example, bypolymerizing a hydrophilic monomer having an acrylic acid or a saltthereof as a main component and which, on exposure to water, absorbs alarge volume of water, preferably absorbs physiological saline withoutload in an amount of 10 to 100 times and forms a substantiallywater-insoluble hydrogel. The water-absorbent resins of this classinclude partially neutralized cross-linked polymers of polyacrylic acid(U.S. Pat. No. 4,625,001, U.S. Pat. No. 4,654,039, U.S. Pat. No.5,250,640, U.S. Pat. No. 5,275,773, EP-A-456136, etc.), a cross-linkedpartially neutralized starch-acrylic acid graft polymer (U.S. Pat. No.4,076,663), a cross-linked isobutylene-maleic acid copolymer (U.S. Pat.No. 4,389,513), a saponified vinyl acetate-acrylic acid ester copolymer(U.S. Pat. No. 4,124,748), a hydrolyzed acrylamide (co)polymer (U.S.Pat. No. 3,959,569), and a hydrolyzed acrylontrile polymer (U.S. Pat.No. 3,935,099), cross-linked carboxymethyl cellulose (U.S. Pat. No.4,650,716 and U.S. Pat. No. 4,689,408), for example. Among otherwater-absorbent resins cited above, the cross-linked polymer of apolyacrylic acid salt proves particularly desirable. The cross-linkedpolymer of the polyacrylic acid is preferable to be such that 50 to 90mol % of the acid group in the polymer is neutralized. As typicalexamples of the salt, alkali metal salts, ammonium salts, and aminesalts may be cited.

[0036] When the water-absorbent resin is obtained by polymerizing ahydrophilic monomer having acrylic acid and/or a salt thereof as a maincomponent thereof, for example, the acrylic acid or the salt thereof maybe copolymerized with other monomer (U.S. Pat. No. 5,338,810 andEP-0574260). As typical examples of the monomer usable for thecopolymerization, anionically unsaturated monomers such as methacrylicacid, maleic acid, β-acryloyloxy propionic acid, vinyl sulfonic acid,styrene sulfonic acid, 2-(meth)acrylamide-2-methyl propanesulfonic acid,2-(meth)acryloyl ethanesulfonic acid, and 2-(meth)acryloylpropanesulfonic acid and salts thereof; nonionic hydrophilicgroup-containing unsaturated monomers such as acrylamide,methacrylamide, N-ethyl(meth)acrylamide, N-n-propyl (meth)acrylamide,N-isopropyl(meth)acrylamide, N,N-dimethyl (meth)acrylamide,2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycolmono(meth)acrylate, vinyl pyridine, N-vinyl pyrrolidone, N-acryloylpiperidine, and N-acryloyl pyrrolidine; and cationically unsaturatedmonomers such as N,N-dimethyl amino-ethyl(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-dimethyl aminopropyl(meth)acrylate,N,N-dimethyl aminopropyl (meth)acrylamide, and quaternary salts thereofmay be cited. The amount of the monomer to be used is generally in therange of from 0 to 50 mol %, preferably from 0 to 30 mol %, mostpreferably from 0 to 10 mol %, based on the amount of all the componentsmonomers for the copolymerization.

[0037] As the water-absorbent resin for use in this invention, althoughit has a cross-linking structure, the water-absorbent resin obtained bythe copolymerization or the reaction using an inner cross-linking agenthaving not less than two polymerizing unsaturated groups or not lessthan two reacting groups or both of polymerizing unsaturated group andreactive group is more preferable than the absorbent resin of theself-cross-linking type which has no use for a cross-linking agent.

[0038] As typical examples of the inner cross-linking agent,N,N′-methylene-bis(meth)acrylamide, (poly)-ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylol-propanetri(meth)acrylate, trimethylolpropane di(meth) acrylate, glyceroltri(meth)acrylate, glycerol acrylate methacrylate, ethyleneoxide-modified trimethylol propane tri(meth)acrylate, pentaerythritoltetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, triallylcyanurate, triallyl isocyanurate, triallyl phosphate, triallyl amine,poly (meth)allyloxy alkane, (poly)ethylene glycol diglycidyl ether,glycerol diglycidyl ether, ethylene glycol, polyethylene glycol,propylene glycol, glycerol, pentaerythritol, ethylene diamine,polyethylene imine, ethylene carbonate, and glycidyl(meth)acrylate maybe cited. These internal cross-linking agents, when necessary, may beused in the form of a combination of two or more members. From theviewpoint of the water-absorption characteristics of the producedwater-absorbent resin, it is particularly preferable to use essentiallya compound having not less than two polymerizable unsaturated groups asan inner cross-linking agent. The amount of the inner cross-linkingagent to be used is preferable to be in the range of from 0.005 to 2 mol%, preferably from 0.01 to 1 mol %, based on the monomer componentsmentioned above.

[0039] The monomers to be polymerized may incorporate therein ahydrophilic polymeric compound such as starch or cellulose, starchderivatives or cellulose derivatives, polyvinyl alcohol, polyacrylicacid (salt), or cross-linked polyacrylic acid (salt), a chain transferagent such as hypophosphorous acid (salt), surfactants, and foamingagents such as carbonates, etc.

[0040] The compounds for addition to these monomers are disclosed inU.S. Pat. No. 4,076,663, U.S. Pat. No. 4,286,082, U.S. Pat. No.4,320,040, U.S. Pat. No. 4,833,222, U.S. Pat. No. 5,118,719, U.S. Pat.No. 5,149,750, U.S. Pat. No. 5,154,713, U.S. Pat. No. 5,264,495,EP-A-03729831, and EP-A-0496594, for example.

[0041] In the polymerization of the monomer for the production of thewater-absorbent resin for use in this invention, though bulkpolymerization or precipitation polymerization is available, it ispreferable to prepare the monomer in the form of an aqueous solution andsubjecting the aqueous solution to solution polymerization orreversed-phase suspension polymerization from the viewpoint of thequality of product and the ease of control of polymerization.Concentration of the aqueous solution is in the range of from 10 to 70%by weight, preferably from 20% by weight to a saturated concentration.

[0042] Such polymerization methods are described in, for example, U.S.Pat. No. 4,625,001, U.S. Pat. No. 4,769,427, U.S. Pat. No. 4,873,299,U.S. Pat. No. 4,093,776, U.S. Pat. No. 4,367,323, U.S. Pat. No.4,446,261, U.S. Pat. No. 4,552,938, U.S. Pat. No. 4,654,393, U.S. Pat.No. 4,683,274, U.S. Pat. No. 4,690,996, U.S. Pat. No. 4,721,647,US-A4,738,867, U.S. Pat. No. 4,748,076, U.S. Pat. No. 4,985,514, U.S.Pat. No. 5,124,416, U.S. Pat. No. 5,247,225 and U.S. Pat. No. 5,250,640.

[0043] The polymerization may be initiated by the use of a radicalpolymerization initiator such as potassium persulfate, ammoniumpersulfate, sodium persulfate, t-butyl hydroperoxide, hydrogen peroxide,or 2,2′-azo-bis-(2-amidinopropane) dihydrochloride or by dint of anactive energy radiation such as ultraviolet light or an electron ray,but it is preferable to use the radical polymerization initiator. Whenan oxidizing radical polymerization initiator is used, such a reducingagent as sodium sulfite, sodium hydrogen sulfite, ferric sulfate, orL-ascorbic acid (salt) may be additionally used to effect redoxpolymerization. These polymerization initiators and reducing agents maybe used in two kinds or more, and an amount thereof to be used isgenerally in the range of from 0.001 to 2 mol %, preferably from 0.01 to0.5 mol % based on the monomer.

[0044] The gel polymer which results from the polymerization ispreferably dried. This drying may be effected by any of the knownmethods such as hot air drying, drying with specific steam (U.S. Pat.No. 4,920,202), microwave drying (U.S. Pat. No. 5,075,344), infrareddrying, vacuum drying, drying by means of a drum drier, and azeotropicdehydration in a hydrophobic organic solvent. The drying is effected ata temperature in the range of from 70° to 300° C., preferably from 1000to 250° C. The gel polymer may be finely divided prior to the drying bythe method disclosed in U.S. Pat. No. 5,275,773. The supply of the gelpolymer to the drier may be controlled by the method taught in U.S. Pat.No. 5,229,487.

[0045] To be used advantageously for this invention, the water-absorbentresin obtained by the polymerization described above assumes the shapeof irregular broken particles, beads, sheets, fibers, rods, and roughspheres. For the purpose of obtaining a water-absorbent agent whichmanifests the effect of the invention to the fullest possible extent andexhibits high absorption capacity under load as without load and excelsin safety owing to the absence of an unaltered residue of across-linking agent, it is preferable to use as the starting rawmaterial spherical or irregular broken water-absobent resin, mostpreferably a water-absorbent resin which is obtained by thepolymerization of an aqueous solution of a monomer and which comprisesirregular broken particles of an average particle diameter of from 200to 600 μm and includes not more than 10% by weight of a fraction ofparticles of less than 150 μm in diameter. If the average particlediameter is less than 200 μm, the absorption capacity under loadpossibly will not be easily improved. Conversely, if it exceeds 600 μm,possibly the speed of absorption will be unduly low and the saturationof absorption will take an unduly long time. It should be noted that ifthe amount of the fraction of resin having particle diameters of lessthan 150 μm exceeds 10% by -weight, the decrease of the amount of aresidue of the cross-linking agent possibly will not be easily attained.

[0046] The water-absorbent resin to be used in this invention can behandled as a powder when it has a water content in the range of from 1to 50%, desirably from 1 to 20%, and more preferably from 1 to 10% afterthe resin resulting from polymerization has been dried as occasiondemands. If the water content exceeds 50%, the cross-linking agent to beused in this invention will permeate the water-absorbent resin particlesthoroughly to the cores and, in spite of the use of the at least oneadditive selected from the group consisting of inorganic acids, organicacids, and polyamino acids, the absorption capacity will be degraded andthe absorption characteristics under load possibly will not be improved.

[0047] The additive to be used in this invention has no particularrestriction except for the requirement that it be an inorganic acidand/or an organic acid and/or a polyamino acid which is soluble in anaqueous liquid. The term “aqueous liquid” as used herein refers to waterand a mixture of water with a hydrophilic organic solvent. Onlynaturally, the substances which have pKa values exceeding 7.0 and,therefore, are uniquely recognized as basic compounds or salts bypersons of ordinary skill in the art do not belong to the inorganic orthe organic acids mentioned above.

[0048] The term “organic acids” as used in this invention refers toacidic compounds which are formed of carbon, hydrogen, and oxygenelements and are represented by organic carboxylic acids, organicsulfonic acids, and organic sulfinic acids. Among organic acids citedabove, the organic carboxylic acids prove particularly preferable.Further, in all the organic carboxylic acids, saturated organiccarboxylic acids prove desirably and hydroxy group-containing saturatedorganic carboxylic acids prove particularly preferable.

[0049] Then, the term “inorganic acids” refers to acidic compounds whichare formed of inorganic substances and hydrogen elements. The inorganicsubstances are desired to be inorganic acids of elements of Group 3 inthe Periodic Table of Element inclusive essentially of at least onemember of phosphorus, sulfur, and chlorine, preferably saturatedinorganic acids of elements of Group 3 in the Periodic Table ofElements. The term “saturated inorganic acids” as used in this inventionrefers to inorganic acids having relevant inorganic substances in themost stable state of oxidation where the inorganic substances arecapable of assuming a plurality of states of oxidation. The inorganicacids which contain phosphorus of the positive pentavalent (+5) form,sulfur of the positive hexavalent (+6) form, and chlorine of thenegative monovalent (−1) form are examples.

[0050] As concrete examples of the additive described above, inorganicacids such as pyrophoshoric acid tripolyphosphoric acid, phosphoricacid, sulfuric acid, and hydrochloric acid and organic acids such asanisic acid, benzoic acid, formic acid, valeric acid, citric acid,glyoxylic acid, glycolic acid, glycerin phosphoric acid, glutaric acid,chloroacetic acid, chloropropionic acid, cinnamic acid, succinic acid,acetic acid, tartaric acid, lactic acid, pyruvic acid, fumaric acid,propionic acid, 3-hydroxypropionic acid, malonic acid, butyric acid,isobutyric acid, imidinoacetic acid, malic acid, isothionic acid,citraconic acid, adipic acid, itaconic acid, crotonic acid, oxalic acid,salicylic acid, gluconic acid, gallic acid, sorbic acid, gluconic acid,and p-oxybenzoic acid may be cited. If such organic or inorganic acid issubstituted by a basic compound or a salt, the absorptioncharacteristics under load will not be improved at all and the decreaseof the amount of a residue of the cross-linking agent used forcross-linking the surface region will be unduly small. Further, even ifthe organic or inorganic salt is added prior to the polymerization, itis meaningless.

[0051] The inorganic acid and the organic acid are preferable to be suchthat the acid dissociation index (the pKa value which is the logarithmof the reciprocal of the acid dissociation constant) thereof in anaqueous solution may be in the range of from 2.0 to 4.0. For example,one or more members selected from the group consisting of phosphoricacid, pyrophosphoric acid, tripoly-phosphoric acid, citric acid,succinic acid, and lactic acid are favorably used. In all the additiveswhich have acid dissociation indexes (pKa values) in the range of from2.0 to 4.0, and among them, organic acids prove preferable. Further, amolecular weight of the organic acids and inorganic acids are preferablein the range of 30 to 1000, more preferably 60 to 500. Among theabove-mentioned saturated organic acids and inorganic acids of elementsof Group 3 in the Periodic Table of Element, such saturated organiccarboxlic acids as succinic acid, citric acid, and lactic acid,especially hydroxyl group-containing saturated organic carboxylic acidsprove more preferable. The acid dissociation index is described in BasicVolume, Chemical Handbook, 3rd edition, pp. II-337 to 342, published byMaruzen Book Co., Ltd. and the pieces of literature indicated in thefootnote thereof.

[0052] In the surface cross-linkage contemplated by this invention, apolyamino acid is favorably used besides the organic acid and theinorganic acid described above to accomplish the desire to decrease theamount of a residue of the cross-linking agent and improve theabsorption capacity under load. From the viewpoint of effectivereduction of the amount of a residue of the cross-linking agent whichconstitutes one of the objects of this invention, the saturated organiccarboxylic acids mentioned above excel polyamino acids. The essentialuse of a polyamino acid, however, brings about prominent effects ofnotably improving the absorbent resin's ability to absorb blood, etc.,as will be described specifically afterward, separately of the effect ofthe inorganic and the organic acid.

[0053] As examples of the polyamino acid usable effectively in thisinvention, homopolymers of at least dimeric order and copolymers whichhave such a-amino acids, p-amino acids, and y-amino acids as asparticacid and lysine coupled through the medium of peptide bonds, withoutreference particularly to distinction between the D form and the L form,may be cited. More specifically, polyglycine, polyalanine, polyvaline,polyleucine, polyisoleucine, polyphenylalanine, polythyrosin,poly-oxyproline, polyproline, polyserine, polythreonine, polythyrosin,polycystein, polycystine, polymethionine, polytryptophan, poly-asparticacid, polyglutamic acid, polylysine, polyarginin, polyhistidine andcopolymers thereof may be cited. Among other polyamino acids (salts)cited above, poly(monoaminodicarboxylic acids) such as polyaspartic acidand polyglutamic acid prove most preferable, and these carboxylic groupand amino group may be partially converted into salts. As respects themolecular weight of the polyamino acid which is usable in thisinvention, those polyamino acids (salts) of relatively low molecularweights of the levels of dimers, desirably pentamers, and more desirablydecamers or the like otherwise called polypeptides up to those polyaminoacids (salts) of very high molecular weights of several millions can beadvantageously used. The molecular weight is desired to be in theapproximate range of from 100 to 1,000,000, preferably from 1,000 to500,000.

[0054] The amount of the additive to be used in this invention isgenerally in the range of from 0.005 to 8 parts by weight, preferablyfrom 0.01 to 5 parts by weight, based on 100 parts by weight of thesolids of the water-absorbent resin, though variable with thecross-linking agent to be used for cross-linking the surface region ofthe absorbent resin particles. If this amount exceeds 8 parts by weight,the excess will neither bring about any economic benefit nor contributeto the accomplishment of the optimum effect of cross-linkage aimed at bythis invention. Conversely, if the amount is less than 0.005 part byweight, the additive will not be fully effective in improving theabsorption characteristics under load or in decreasing the amount of aresidue of the cross-linking agent. The most preferable amount is in therange of from 0.1 to 3 parts by weight. In the case of a polyamino acid,the amount for effective use may be increased up to 30 parts by weightfor the reason which will be described hereinafter.

[0055] The expression “cross-linking agent capable of reacting with acarboxyl group” and usable in this invention means that thiscross-linking agent reacts with a carboxyl group by forming a covalentbond or an ionic bond therewith. Numerous well-known cross-linkingagents in popular use answer this description. As concrete examples ofthe cross-linking agent, polyhydric alcohol compounds such as ethyleneglycol, diethylene glycol, propylene glycol, triethylene glycol,tetraethylene glycol, polyethylene glycol, propylene glycol,1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentadiol,polypropylene glycol, glycerol, polyglycerol, 2-butene-1,4-diol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylol propane, diethanol amine,triethanol amine, polyoxypropylene, oxyethylene-oxypropylene blockcopolymer, pentaerythritol, and sorbitol; epoxy compounds such asethylene glycol diglycidyl ether, polyethylene diglycidyl ether,glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerolpolyglycidyl ether, propylene glycol diglycidyl ether, polypropyleneglycol diglycidyl ether, and glycidol; polyamine compounds such asethylene diamine, diethylene triamine, triethylene tetramine,tetraethylene pentamine, pentaethylene hexamine, polyamide polyamine,and polyethylene imine and epihalohydrin condensates with thesepolyamine compounds; polyisocyanate compounds such as 2,4-tolylenediisocyanate and hexamethylene diisocyanate; polyoxazoline compoundssuch as 1,2-ethylene-bis-oxazoline; silane coupling agents such asγ-glycidoxypropyl trimethoxy silane and γ-amino propryl trimethoxysilane; alkylene carbonate compounds such as 1,3-dioxolan-2-on,4-methyl-1,3-dioxolan-2-on, 4,5-dimethyl-1,3-dioxolan-2-on,4,4-dimethyl-1,3-dioxolan-2-on, 4-ethyl-1,3-dioxolan-2-on,4-hydroxymethyl-1,3-dioxolan-2-on, 1,3-dioxan-2-on,4-methyl-1,3-dioxan-2-on, 4,6-dimethyl-1,3-dioxan-2-on, and1,3-dioxolan-2-on; haloepoxy compounds such as epichlorohydrin,epibromohydrin, and α-methyl epichlorohydrin; and polyvalent metalcompounds such as hydroxides and chlorides of zinc, calcium, magnesium,aluminum, iron, and zirconium may be cited. These cross-linking agentsmay be used either singly or in the form of a mixture of two or moremembers. And among these cross-linking agents, at least one memberselected from the group consisting of the polyvalent alcohols, epoxycompounds, alkylene carbonate compound, polyamine compounds andepihalohidrin condensate thereof are preferable.

[0056] For the purpose of attaining the effects of this invention to thefullest possible extent, it is desirable to use an epoxy compound as anessential component of the cross-linking agent. In this case, theeffects of this invention may be manifested more conspicuously by usingtwo or more kinds of cross-linking agents having solubility parametersin different ranges as shown in EP Application No. 93101068.5 and U.S.Ser. No. 08/167,077.

[0057] When at least two kinds of the cross-linking agents are used, afirst cross-linking agent which can preferably be used in this inventionis required to possess a solubility parameter (SP value) of 12.5(cal/cm³)^(½) and exhibit reactivity with a carboxyl group. Thesolubility parameter (SP value) is used generally as a factorrepresenting the polarity of a given compound. This invention adopts thenumerical value of the solubility parameter δ (cal/cm³)^(½) of solventreported in the “Polymer Handbook,” third edition (Wiley InterscienceCorp), VII-527 to 539. For a cross-linking agent which is not includedin the table, this invention proposes to adopt the numerical value δ(cal/cm³)^(½) which is derived by substituting the Hoy's cohesive energyconstant reported in VII-525 ibidem in the Small's formula found inVII-524 ibidem.

[0058] As typical examples of the first cross-linking agent whichpossesses such a solubility parameter (SP value) of not less than 12.5[(cal/cm³)^(½)] and exhibits reactivity with a carboxyl group, ethyleneglycol, propylene glycol, glycerol, polyglycerol, pentaerythritol,sorbitol, ethylene carbonate, propylene carbonate, etc. may be cited.One member or a mixture of two or more members selected from among theseexamples may be used effectively. Preferably, this solubility parameter(SP value) is in the range of 13.0 to 18.0 (cal/cm³)^(½).

[0059] When at least two kinds of the cross-linking agents are used, asecond cross-linking agent which is effectively usable in this inventionis preferable to possess a solubility parameter (SP value) of less than12.5 (cal/cm³)^(½) and exhibit reactivity with a carboxyl group. Astypical examples of the second cross-linking agent, diethylene glycol,triethylene glycol, tetraethylene glycol, dipropylene glycol,tripropylene glycol, 1,3-butanediol, 1,4-butane-diol, 1,5-pentanediol,2,4-pentanediol, 1,6-hexanediol, 2,5-hexanediol, trimethylol propane,diethanolamine, triethanolamine, ethylene glycol diglycidyl ether,polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether,diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether,propylene glycol diglycidyl ether, polypropylene glycol diglycidylether, ethylene diamine, diethylenetriamine, triethylenetetramine,2,4-tolylene diisocyanate, hexamethylene diisocyanate,4,5-dimethyl-1,3-dioxolan-2-on, epichlorohydrin, and epibromohydrin maybe cited. One member or a mixture of two or more members selected fromamong these cross-linking agents can be used effectively herein.Preferably, the solubility parameter (SP value) is in the range betweennot less than 9.5 (cal/cm³)^(½) and less than 12.0 (cal/cm³)^(½).

[0060] When at least two kinds of the cross-linking agents are used, theselection of the first and second cross-linking agents is preferable tobe such that the difference between the solubility parameter (SP value)of the first cross-linking agent and the solubility parameter (SP value)of the second cross-linking agent is not less than 2 (cal/cm³)^(½). Ifthe difference of the two solubility parameters (SP values) is less than2 (cal/cm³)^(½), the possibility arises that the effect to be derivedfrom the use of these two species of cross-linking agent will not beeasily manifested and the improvement of absorption properties underload will be attained with difficulty. Preferably, the selection of thefirst and second cross-linking agents is such that the difference of thesolubility parameter (SP value) of the first cross-linking agent and thesolubility parameter (SP value) of the second cross-linking agent is notless than 3 (cal/cm³)^(½).

[0061] The first cross-linking agent is preferable to have a molecularweight of not more than 200 and the second cross-linking agent to have amolecular weight of not more than 350. If the molecular weight of thefirst cross-linking agent exceeds 200, the possibility ensues that thefirst cross-linking agent will not permeate the particles of theabsorbent resin as the base polymer deeply through the surface thereofbut tend to stagnate in the surface region thereof and the absorptionproperties of the produced absorbent resin under load will be improvedwith difficulty. If the molecular weight of the second cross-linkingagent exceeds 350, the possibility arises that this second cross-linkingagent will only sparingly permeate the particles of the absorbent resinas the base polymer and the absorption properties of the producedabsorbent resin under load will be improved only with difficulty.Preferably, the first cross-linking agent has a molecular weight in therange of 50 to 150 and the second cross-linking agent a molecular weightin the range of 150 to 300.

[0062] In the present invention, the additive is effective in optimizingthe distribution of the cross-linking agent, enabling the producedabsorbent agent to exhibit a high absorption capacity under load,decreasing the amount of a residue of the cross-linking agent. When anepoxy compound abounding in reactivity is used as the additive, theeffect in decreasing the amount of a residue of the cross-linking agentis especially prominent. Thus, the water-absorbent agent of high safetywhich excels in absorption characteristics and befits sanitary materialscan be obtained with high efficiency from the viewpoint of process.

[0063] The amount of the cross-linking agent to be used in thisinvention is generally in the range of from 0.001 to 10 parts by weight,preferably from 0.005 to 8 parts by weight, more preferably from 0.01 to5 parts by weight, and most preferably from 0.02 to 2 parts by weight,based on 100 parts by weight of the solids of the water-absorbent resinof this invention, through variable with the kind of cross-linking agentto be used. So long as the amount of the cross-linking falls within therange specified above, the produced water-absorbent resin exhibits fineabsorption characteristics under load. If this amount exceeds 10 partsby weight, the excess will neither bring about any economic advantagenor make any contribution to the accomplishment of the proper effect ofcross-linkage but tend to give rise to a residue in spite of the use ofthe additive of this invention. Conversely, if this amount is less than0.001 part by weight, then the cross-linking agent will not be amplyeffective in exalting absorption characteristics under load.

[0064] Further, when at least two kinds of the cross-linking agents areused, the use both first and second cross-linking agents mentioned aboveis preferable, and in such case, the amounts of the first and secondcross-linking agents to be used in this invention are variable with theparticular kinds of these cross-linking agents to be adopted. Generally,the amount of the first cross-linking agent is in the range of 0.001 to10 parts by weight and that of the second cross-linking agent in therange of 0.001 to 10 parts by weight, respectively based on 100 parts byweight of the solids of the absorbent resin as the base polymer. Theamount of the first cross-linking agent is preferably in the range of0.01 to 8 parts by weight, more preferably 0.1 to 5 parts by weight andthat of the second cross-linking agent is preferably in the range of0.001 to 1 part by weight, more preferably 0.005 to 0.5 part by weight.

[0065] In this invention, it is preferable to use water in mixing thewater-absorbent resin and the cross-linking agent. The amount of waterthus used in this invention is generally in the range of from 0.5 to 5parts by weight, preferably from 0.5 to 3 parts by weight, based on 100parts by weight of the solids of the absorbent resin, though variablewith the kind, particle size, and water content of the water-absorbentresin. If the amount of water to be used exceeds 5 parts by weight, theimprovement of absorption characteristics under load possibly will notbe easily attained and the cross-linking agent will possibly tend togive rise to a residue in spite of the use of the additive of thisinvention. Conversely, if the amount is less than 0.5 part by weight,the effective improvement of absorption characteristics under loadpossibly will not be easily attained.

[0066] In this invention, the mixture of the absorbent resin with thecross-linking agent may be effected through the medium of a hydrophilicorganic solvent. As concrete examples of the hydrophilic organic solventwhich is effectively usable herein, lower alcohols such as methylalcohol, ethyl alcohol, n-propyl alcohol, iso-propyl alcohol, n-butylalcohol, isobutyl alcohol, and t-butyl alcohol; ketones such as acetone;ethers such as dioxane, alkoxy(poly)ethylene glycols, andtetrahydrofuran; amides such as N,N-dimethyl formamide; and sulfoxidessuch as dimethyl sulfoxide may be cited. The amount of the hydrophilicorganic solvent to be used in this invention generally is in the rangeof from 0 to 10 parts by weight, preferably from 0.1 to 5 parts byweight, based on 100 parts by weight of the solids of thewater-absorbent resin, though variable with the kind and the particlesize of the water-absorbent resin.

[0067] In this invention, the mixing of the water-absorbent resin withthe cross-linking agent and the additive may be carried out in such astate as has the water-absorbent resin dispersed in an organic solventlike cyclohexane or pentane. The invention is manifested to the fullestpossible extent. To be specific, this mixture is preferably accomplishedby:

[0068] 1. A method which comprises preparatorily mixing a cross-linkingagent optionally containing water and/or a hydrophilic organic solventwith an additive and subsequently adding the resultant mixture to anabsorbent resin by either direct spraying or dropwise incorporation.

[0069] 2. A method which comprises preparatorily mixing awater-absorbent resin with an additive and subsequently adding to theresultant mixture a cross-linking agent optionally containing waterand/or a hydrophilic organic solvent by either spraying or dropwiseincorporation.

[0070] 3. A method which comprises directly adding a cross-linking agentoptionally containing water and/or a hydrophilic organic solvent to awater-absorbent resin by either spraying or dropwise incorporation andfurther mixing an additive therewith.

[0071] 4. A method which comprises spraying or mixing dropwise theadditive optionally with the cross-linking agent containing water and/orhydrophilic organic solvent parallely by two nozzes.

[0072] From the standpoint of absorption characteristics and theeffective decrease of the amount of a residue of the cross-linkingagent, the method 1 mentioned above proves preferable. When the mixtureis implemented through the medium of water, it may be carried out in thepresence of a water-insoluble finely divided powder or a surfactant.

[0073] The device to be used for the mixture must be capable ofgenerating a large mixing force enough to enable the mixture to proceeduniformly. As concrete examples of the mixing device to be effectivelyusable in this invention, cylindrical mixers, double-wall conicalmixers, high-speed stirring type mixers, V-shaped mixers, ribbon typemixers, screw type mixers, fluidized-bed oven rotary disk type mixers,air current type mixers, twin-arms type kneaders, crushing typekneaders, rotary type mixers, and screw type extruders may be cited.

[0074] This invention contemplates a procedure which comprises mixing awater-absorbent resin with a cross-linking agent, preferably with boththe cross-linking agent and an additive simultaneously by theabove-mentioned method 1 and then preferably further heat-treating theresultant mixture depending on the kind of the cross-linking agentthereby cross-linking the surface region of the adhesive resinparticles.

[0075] When this invention adopts the heat treatment, the temperature ofthis heat treatment is desired to be in the range of from 80 to 300° C.If the temperature of the heat treatment is lower than 80° C., the heattreatment will consume time so much as to induce degradation ofproductivity, obstruct uniform progress of cross-linkage, possiblyprevent production of a resin of high absorption characteristics underload, and suffer the cross-linking agent to give rise to a residue. Thetemperature of the heat treatment is preferably in the range of from 100to 230° C., more preferably from 160 to 220° C., though variable withthe kind of cross-linking agent to be used.

[0076] The heat treatment can be performed by the use of a standarddrier or a standard heating furnace. As typical examples of the heatingdevice effectively usable herein, groove type combination mixers anddriers, rotary driers, dick driers, fluidized-bed type driers, aircurrent type driers, and infrared driers may be cited.

[0077] The water-absorbent agent which is obtained by the method ofproduction of this invention described above is a product of theaddition of from 0.005 to 8% by weight of an epoxy compound to awater-absorbent resin containing carboxyl group. It is water-absorptionagent which exhibits an absorption capacity wihtout load of not lessthan 45 (g/g) and an absorption capacity under load of 20 g/cm² of notless than 30 (ml/g), preferably not less than 35 (ml/g), and possesses aresidue amount of an epoxy compound concentration of not more than 2ppm, preferably 0 ppm. The water-absorbent agent of this inventionpreferably contains from 0.01 to 5% by weight of a saturated organicacid, preferably a hydroxy group containing saturated organic acid inrespect of safety and absorption properties.

[0078] Heretofore, when the cross-linking agent such as an epoxycompound is used for surface cross-linking, it has inevitably entailedthe occurrence of a residue of the order of from some tens to 1,000 ppm.The water-absorbent agent which is obtained by this invention exhibitsoutstanding absorption characteristics both without load and under loadand reveals a notable decrease of the amount of a residue of across-linking agent of high reactivity on the resin surface. The causefor these advantages of this invention has not been clearly defined.They may be logically explained by a postulate that when thewater-absorbent resin is mixed the cross-linking agent with the additiveof this invention, the additive discharges the role of optimizing thedistribution of the cross-linking agent in the surface region of thewater-absorbent resin particles and, at the same time, accelerating anduniformizing the reaction between the absorbent resin and thecross-linking agent and consequently notably decreasing the amount of aresidue of the cross-linking agent and enabling the cross-linkage ofuniform density gradient to be formed in the surface region of thewater-absorbent resin particles. When the water-absorbent resin isproduced in the form of irregular broken particles as mentionedpreviously, it enjoys such advantages as high safety, thorough diffusionof liquid, and perfect absence of migration or exfoliation of resinparticles from pulp in addition to the excellent absorptioncharacteristics mentioned above. Thus, this absorbent resin isparticularly adapted for absorbent products such as disposable diaperand sanitary materials.

[0079] This invention also provides a novel water-absorbent compositionwhich comprises a polyamino acid (salt) and a carboxyl group-containingwater-absorbent resin. As the polyamino acid(salt) for use in thiswater-absorbent composition, the polyamino acids previously cited asadditives for the surface cross-linkage of the absorbent resin mentionedabove are available. Preferably, in addition to suchpoly(monoaminodicarboxylic acids) as polyaspartic acid and polyglutamicacid which are advantageously used for the surface cross-linkage,polylysine and polyarginine and more desirably suchpoly(diaminomonocarboxylic acids) and salts thereof as polylysine areused favorably in the composition of this invention.

[0080] The water-absorbent composition of this invention is essentiallycomposed of the absorbent resin mentioned above and the polyamino acid(salt). Preferably, the ratio of the polyamino acid (salt) is in therange of from 0.005 to 30 parts by weight, preferably from 0.005 to 8parts by weight, most preferably from 0.01 to 5 parts by weight, to 100parts by weight of the water-absorbent resin. If the amount of thepolyamino acid (salt) is smaller than the lower limit of the rangementioned above, the effect of this invention will not be produced.Conversely, it is larger than the upper limit of the range, the excesswill not bring about a proportional increase to the effect but willpossibly degrade the absorption quality.

[0081] The water-absorbent composition of this invention is favorablyobtained by a method which essentially uses the polyamino acid duringthe surface cross-linkage of the water-absorbent resin described above.In this case, the water-absorbent composition containing the polyaminoacid (salt) is formed. The acid type polyamino acid is ideally used fordecreasing the amount of a residue of the cross-linking agent. For thepurpose of improving the absorption of blood which constitutes anotherobject of this invention, substantially the same effect can be attainedwith the absorbent composition incorporating therein an amino polyacidsalt.

[0082] For the production of the water-absorbent composition of thisinvention, therefore, a method which consists in effecting the additionof the polyamino acid (salt) in the process of the formation of thewater-absorbent resin may be adopted besides the method which resorts tothe surface cross-linkage. Specifically, the polyamino acid (salt) maybe added to the aqueous monomer solution prior to polymerization or tothe reaction solution during or after polymerization. Otherwise, amethod which resides in causing the polyamino acid (salt) to be added tothe water-absorbent resin after the resin has been formed may beadopted. For the addition of the polyamino acid (salt) to thewater-absorbent resin in the dry state, the so-called dry blend methodwhich comprises mechanically mixing the two components both in apowdered state or the method which effects the mixture by adding eitheror both of the two components in the form of an aqueous dispersion oraqueous solution. Among other methods cited above, the method whichresides in adding the polyamino acid (salt) in the form of an aqueousliquid or a dispersion, preferably the aqueous liquid to thewater-absorbent resin in a dry state proves particularly preferable inrespect that the produced water-absorbent composition excels in variousproperties. In this case, water alone or a mixture of water with ahydrophilic organic solvent can be used as the aqueous liquid.

[0083] The amount of the aqueous liquid to be used is in the range offrom 0.1 to 50 parts by weight, preferably from 1 to 30 parts by weight,based on 100 parts by weight of the solid of the water-absorbent resin.As typical examples of the hydrophilic organic solvent to be used in themixture mentioned above, lower alcohols such as methyl alcohol, ethylalcohol, n-propyl alcohol, iso-propyl alcohol, n-butyl alcohol,iso-butyl alcohol, and t-butyl alcohol; ketones such as acetone; etherssuch as dioxane, alkoxy(poly)ethylene glycol and tetrahydrofuran; amidessuch as N,N-dimethyl formamide; and sulfoxides such as dimethylsulfoxide may be cited.

[0084] Further in this invention, the water-absorbent composition havingthe polyamino acid (salt) added in the form of an aqueous liquid to thewater-absorbent resin is subjected to a heat treatment when necessary.The temperature of this heat treatment is generally in the range of from500 to 300° C., preferably 1000 to 230° C., though variable with theamount of the aqueous liquid to be added. The heat treatment is desiredto be effected under conditions such that the water-absorbentcomposition of this invention may be obtained in a substantially driedstate.

[0085] When the water-absorbent resin is mixed with the polyamino acidin the aqueous liquid as described above, particularly by causing across-linking agent capable of reacting with the carboxyl group to bepreparatorily contained in the aqueous liquid and as above-mentionedheat-treating the resultant mixture thereby introducing a cross-linkedstructure in the surface region of the water-absorbent resin, thewater-absorbent composition to be produced by this invention is enabledto acquire an ability to aspirate physiological saline solution andblood and excel particularly in absorption capacity under load.

[0086] The water-absorbent composition of this invention obtained asdescribed above is a novel water-absorbent composition which exhibitsoutstanding absorption characteristics to such body liquids as urine,blood, and excretion, manifests fine gel stability, high flowability(anti-blocking property) after absorbing moisture, static charge, andthe rustproofing effect.

[0087] This invention has brought to light a novel water-absorbentcomposition which exhibits an excellent suction power to physiologicalsaline solution and bovine blood. Heretofore, the fact that thewater-absorbent agent having an aspirating capacity of not less than 14g/g of artificial urine, when used in a disposable diaper, is capable ofrepressing leakage (JP-A-63-21,902) and the absorbent material whichexhibits an excellent absorption capacity to blood (JPA-06-25,543) havebeen known to the art. These known techniques do not necessarilygenerate a fully satisfactory aspirating capacity on such highly viscousliquids as blood, menstrual fluid, and loose feces. The presentinvention offers a water-absorbent composition which has overcome theheretofore insoluble problems. It has been ascertained by this inventionthat for sanitary napkins and disposable diapers using a water-absorbentcomposition at high concentrations (the absorbent structures containingthe absorption composition at concentrations of not less than 50% byweight), the balance among the absorption capacity without load and thesuction power exhibited to physiological saline solution and the suctionpower exhibited to bovine blood, i.e. the characteristic featurepresented by the water-absorbent composition of this invention,constitutes an important requirement.

[0088] Thus, this invention provides a water-absorbent composition whichexhibits an absorption capacity without load of at least 25 g/g tophysiological saline solution and a suction power of not less than 14g/g to physiological saline solution and a suction power of at least 7g/g to bovine blood and also provides a water-absorbent structure and awater-absorbent article both incorporating therein the absorbentcomposition or the absorbent agent mentioned above.

[0089] The suction power of the water-absorbent composition exhibited tophysiological saline solution and that exhibited to bovine bloodconstitute essential factors for the acquisition of such awater-absorbent article as disposable diaper or sanitary napkin orsimilar sanitary material which quickly absorbs urine or blood diffusedin a hydrophilic fibrous material supporting the water-absorbentcomposition, retains the diffusing property of the hydrophilic fibrousmaterial stably to withstand the effect of aging, avoids inflicting anoffensive sensation to the skin, and represses the amount of urine orblood suffered to flow back. It has been ascertained particularly thatthe ratio of the absorbent composition incorporated in thewater-absorbent structure is allowed to be notably increased (to 50% byweight or even higher) when the suction power exhibited to bovine bloodexceeds 7 g/g. The cause for this feature is not clear. The feature maybe logically explained, however, by a supposition that the proportion ofblocks generated between particles owing to the high concentration ofthe water-absorbent composition in the water-absorbent structure isincreased when the water-absorbent structure is used on such a viscousliquid as blood. Thus, it is inferred that the differences brought outin absorption characteristics exhibited to bovine blood play animportant role.

[0090] The absorption capacity without load exhibited to physiologicalsaline solution constitutes an important factor in respect that itgoverns the absorption capacity of the water-absorbent article. Thisabsorption capacity without load is such that the suction power and theabsorption ratio under load are decreased in proportion as theabsorption capacity without load is increased. Thus, the absorptioncapacity without load has a relation of antinomy relative to the suctioncapacity and the absorption capacity under load. By causing thewater-absorbent composition of this invention which have the absorptioncapacity without load, the suction power, and the absorption capacityunder load controlled within the specific ranges to be combined at aspecific mixing ratio with the hydrophilic fibers and the waterabsorbent, the absorbent structure and the water-absorbent article ofthis invention which possess the heretofore unattainable absorptioncharacteristics can be obtained.

[0091] The hydrophilic fibers for use in the absorbent structure of thisinvention are desired to be in the form of an airlaid batt of groundwood pulp. The structure of this airlaid batt is well known in the fieldof manufacture of disposable diapers as disclosed in U.S. Pat. No.4,610,678, for example. As typical examples of the hydrophilic fiberswhich are effectively usable for this invention, cotton linters, suchcross-linked cellulose fibers as are disclosed in EP-A-0429112 andEP-A-0427316, rayon fibers, cotton fibers, wool fibers, acetate fibersand vinylon fibers may be cited besides the ground pulp.

[0092] The water-absorbent structure of this invention is composed offrom 10 to 95 parts by weight of a water-absorbent composition orwater-absorbent agent and from 90 to 5 parts by weight of absorbentfibers, preferably from 50 to 95% by weight of the water-absorbentcomposition and 50 to 5% by weight of the hydrophilic fibers. Thecomposition and the fibers are combined to form a matrix as intermixedor dispersed on a fibrous sheet. Several methods including the methodcurrently adopted for the manufacture of disposable diapers on themarket are available for the formation of the water-absorbent structure.Methods proper for the formation of the absorbent structure are disclsedin U.S. Pat. No. 4,578,068, U.S. Pat. No. 4, 666,975, U.S. Pat. No.4,673,402, U.S. Pat. No. 4,685,915, U.S. Pat. No. 4,765,780, U.S. Pat.No. 5,047,023, and U.S. Pat. No. 5,156,902, for example. Thewater-absorbent structure which is obtained as described above ispreferably compression molded until the density thereof reaches a levelin the range of from 0.06 to 0.5 g/cc. In this invention, therefore, thedensity is preferable to fall in this range because the absorbentstructure which contains the water-absorbent composition or thewater-absorbent agent at a relatively high concentration has a relativesmall thickness and manifests the absorbing power efficiently. If thedensity exceeds the upper limit of this range, the water-absorbentstructure will be unduly hard. Conversely, if the density falls belowthe lower limit of the range, the water-absorbent structure will bedeficient in the efficiency of absorption. The water-absorptionstructure of this invention is preferable to have a basis weight in therange of from 0.01 to 0.20 g/square centimeter. If the basis weight issmaller than the lower limit of the range, the water-absorbent structurewill be deficient in strength. If it is larger than the upper limit ofthe range, the water-absorbent structure possibly will be too hard to befavorably adapted for absorbent articles. The water-absorbent structureof this invention, therefore, is allowed to have an average thicknessless than 40 mm, preferably less than 10 mm, and more preferably lessthan 5 mm.

[0093] In the present invention, the water-absorbent structure whichcomprises the water-absorbent composition or the water-absorbent agentand the hydrophilic fibers is allowed to contain therein up to 40 partsby weight of thermoplastic fibers based on 100 parts by weight of thehydrophilic fibers for the purpose of enabling the water-absorbentstructure to retain the shape thereof during the absorption of water. Astypical examples of the thermoplastic fibers which are effectively usedfor the purpose just mentioned, polyethylene, polypropylene, andpolyester fibers, and bicomponent fibrous copolymers of polyester andpolyamides and complex fibers thereof may be cited.

[0094] The water-absorbent structure of this invention which obtained asdescribed above is nipped fast between a facing member pervious toliquid and a backing member impervious to liquid to produce awater-absorbent article of this invention.

[0095] The water-absorbent article of this invention may be providedwith numerous members such as a diffused layer, an elastic leg partmember, an elastic waist part member, and tapes which are well known inthe technical field to which this invention pertains.

[0096] The water-absorbent article of this invention can be adapted forvarious products such as disposable diapers, athlet's pants, sanitarynapkins, surgical pads, clothing for patients of incontinence, andsheets for pets which are required to absorb large volumes of liquid.

[0097] The water-absorbent agent, the water-absorbent composition, orthe water-absorbent structure which is obtained by this invention may beendowed with a new function by addition of such additives as deodorant,perfume, inorganic powder, foaming agent, pigment, dye, hydrophilicshort fibers, plasticizer, binder, fertilizer, oxidizing agent, reducingagent, water, and salts or may be granulated. Typical examples of theseadditives are cited in U.S. Pat. No. 4,179,367, U.S. Pat. No. 4,190,563,U.S. Pat. No. 4,500,670, U.S. Pat. No. 4,693,713, U.S. Pat. No.4,812,486, U.S. Pat. No. 4,863,989, U.S. Pat. No. 4,929,717, U.S. Pat.No. 4,959,060, U.S. Pat. No. 4,972,019, U.S. Pat. No. 5,078,992, andU.S. Pat. No. 5,229,488, EP-A-0009977 and EP-A-0493011. Typical examplesof the method of granulation are cited in U.S. Pat. No. 4,734,478 andEP-A-0450922 and EP-A-0480031, for example.

[0098] Now, the present invention will be described specifically belowwith reference to working examples. It should be noted, however, thatthese examples are not meant to define the scope of the presentinvention. All “parts” in examples are by weight unless they arespecifically noticed.

[0099] Various properties of the water-absorbent agents indicated in theexamples were determined by the following methods.

[0100] (a) Absorption Capacity to Physiological Saline Without Load

[0101] A teabag pouch of non-woven fabric (40 mm×150 mm) having 0.2 g ofa sample water-absorbent or water-absorbent composition uniformlycontained therein was immersed in an aqueous solution containing excessamount of 0.9% by weight of sodium chloride (physiological salinesolution). After 60 minutes' standing in the saline solution, the teabagpouch containing a swelled gel was pulled out over a period of 5 secondsfrom the saline solution, left draining for 10 seconds on 24 superposedsheets of toilet papers (57 mm×50 mm, 19 g/m² per 1 sheet), and thenweighed (W1). The same procedure was repeated by way of a blank testusing no sample to find the blank weight of the teabag pouch (W0). Theabsorption capacity without load was calculated in accordance with thefollowing formula 1.

Absorption capacity without load to physiological saline (g/g)=[Weight,W ₁(g), after absorption−Blank weight, W ₀(g)]/(Weight of absorbentresin agent (or composition)(g))  (1)

[0102] (b) Absorption Capacity Under Load

[0103] The absorption capacity under load was determined by the use ofthe same device as illustrated in FIGURE. With an upper mouth 2 of aburet 1 closed with a stopper 3, a measuring base 4 and an air inlet 5were set at an equal level. A filter paper 7 was mounted on a glassfilter 6 of a diameter of 70 mm located in the central part of themeasuring base 4. A non-woven fabric 8 was fixed at the lower end partof a supporting cylinder 10 having a diameter of 55 mm. On the non-wovenfabric 8, 0.76 g of a sample water-absorbent agent was uniformly spreadand a load 9 of 20 g/cm² was placed. This set of non-woven fabric,water-absorbent agent, and load as held on the supporting cylinder 10was mounted on the filter paper 7 overlying the glass filter 6. Theamount (A ml) of the aqueous solution of 0.9% by weight of sodiumchloride (physiological saline solution) absorbed over a period of 30minutes was measured. The absorption capacity (ml/g) under load wascalculated in accordance with the following formula 2.

Absorption capacity under load(ml/g)=A(ml)/0.76(g)  (2)

[0104] (c) Amount of Residue of Surface Cross-linking Agent (in the Caseof an Epoxy Compound)

[0105] In a beaker having an inner volume of 100 ml, 2.0 g of a samplewater-absorbent agent was placed and 2 ml of a composed solution ofmethanol/water=2/1% by weight was added thereto and the resultantmixture was left standing for one hour, with the beaker kept closed witha lid. The sample remaining in the beaker was mixed with 5 ml ofmethanol and the resultant mixture was filtered. In an eggplant flaskhaving an inner volume of 50 ml, 1.0 g of the filtrate was placed andcombined with 0.05 ml of an aqueous solution of 12 wt % of nicotineamide. The flask was fitted with an air cooling tube and heated in awater bath for 30 minutes. The reaction solution was passed through afilter paper and the filtrate was assayed by high-performance liquidchromatography.

[0106] Separately, the same procedure was repeated by way of a blanktest using a known amount of cross-linking agent and omitting the use ofa water-absorbent agent to obtain a calibration curve. With thiscalibration curve as an external standard and in due consideration ofthe ratio of dilution of the filtrate, the amount of residue of surfacecross-linking agent on (ppm) in the sample was determined.

[0107] (d) Suction Power to Physiological Saline Solution

[0108] About 1 g of a sample water-absorption agent (or composition) wasplaced on 16 superposed sheets of toilet paper (19 g/m^(2, 55) mm×75 mmper 1 sheet) kept immersed in 20 ml of the physiological saline solutionheld in a petri dish 95 mm in inside diameter and then allowed toaspirate the saline solution for 10 minutes. Then the swelled gelconsequently formed was collected and weighed. The capacity of thesample water-absorbent agent or composition for absorption of thephysiological saline solution (g/g) was calculated by dividing theweight of the swelled gel by the original weight of the water-absorbentagent (or composition).

[0109] (e) Suction Power to Bovine Blood

[0110] The suction power to bovine blood (g/g) was determined byfaithfully repeating the procedure used for the determination of thesuction power to physiological saline solution while using bovine blood[bovine blood liquid prepared by reconstituting a freeze dried productof Wako Pure Chemical Industries, Ltd. marketed under “Wako Code Number029-07071”] in the place of physiological saline solution.

[0111] (f) Flowability (Anti Blocking Property) After Absorbing Moisture

[0112] This property was rated by causing an aluminum cup (50 mm indiameter on the bottom side and 23 mm in height) containing 2 g of asample water-absorbent composition to stand at rest for 1.5 hours in athermo-hygrostat regulated at 25° C. and 50% RH and, after the standing,removing the aluminum cup and tilting it at an angle of 30° and visuallyexamining the condition of flow of the absorbent composition in thealuminum cup.

[0113] (g) Rustproofing Effect and Gel Stability

[0114] This property was determined by placing 1 g of a samplewater-absorbent composition in 25 cc of physiological saline solutionheld in a lidded container of polypropylene measuring 55 mm in diameterand 70 mm in height thereby allowing the sample to swell with the salinesolution, thoroughly immersing an iron clip in the resultant swelledgel, and allowing the iron clip to stand at rest therein for three daysat normal room temperature and, at the end of the standing, visuallyexamining the gel to find whether or not the iron clip produced rust inthe gel. The visual observation of the gel was concurrently utilized forrating the stability of the gel.

[0115] (h) Ability to Prevent Static Charge

[0116] This property was determined by placing 10 g of a samplewater-absorbent composition in a polyethylene bag fitted with a slidefastener (140 mm×100 mm×0.04 mm, produced by Seisan Nippon K.K. andmarketed under trademark designation of “Unipack E-4”), closing the bag,vigorously shaking the bag vertically for one minute, and visuallyexamining the bag to rate the extent of adhesion of the water-absorbentcomposition to the inner walls of the bag.

REFERENTIAL EXAMPLE 1

[0117] In 5500 g an aqueous solution of sodium acrylate having a ratioof neutralization of 75 mol % (monomer concentration 33%), 1.58 g ofN,N′-methylene-bis-acrylamide as a inner cross-linking agent wasdissolved. The resultant solution was deaerated by bubbling nitrogen gastherethrough for 30 minutes and then supplied into a reaction vesselconstructed by lidding a jacketed twin arm type kneader of stainlesssteel having an inner volume of 10 liters and provided with two sigmavanes. The reaction vessel was further subjected to displacement of theentrapped air with nitrogen gas, with the monomer kept at 30° C. When2.4 g of ammonium persulfate and 0.12 g of 1-ascorbic acid were added tothe reaction system with the vanes kept in rotation meanwhile, themonomer therein began to polymerize 1 minute thereafter. After thefurther elapse of 16 minutes thereafter, the inner temperature of thereaction system reached 83° C. and the cross-linked hydrogel polymerproduced meanwhile was finely divided into particles about 5 mm indiameter. Then, the stirring of the contents of the reaction vessel wascontinued. The cross-linked hydrogel polymer was removed from thereaction vessel after the elapse of 60 minutes following the start ofthe polymerization.

[0118] The finely divided particles of the cross-linked hydrogel polymerthus obtained were spread on a metal gauze of ASTM 50-mesh and driedwith hot air at 150° C. for 90 minutes. The dried particles werepulverized with a shaking mill and further classified with an ASTM20-mesh sieve to obtain an irregular broken water-absorbent resin (A)having average diameter of 360 μm and containing resin particles ofdiameters of less than 150 μm at a proportion of 5% by weight and havinga water content of 6% by weight.

REFERENTIAL EXAMPLE 2

[0119] In 5500 g of an aqueous solution of sodium acrylate having aratio of neutralization of 75 mol % (monomer -concentration of 33%), 4.4g of polyethylene glycol diacrylate as the inner cross-linking agent wasdissolved. The resultant solution was deaerated with nitrogen gas andthen combined with 2.4 g of sodium persulfate and 0.12 g of 1-ascorbicacid to set the monomer to polymerizing. At the time that thepolymerization was completed, the cross-linked hydrogel polymer producedin the meanwhile was finely divided and dried in a hot air drier at 150°C. The dried particles of the polymer consequently obtained werepulverized with a roll granulator and classified with an ASTM 20-meshsieve to obtain an irregular broken water-absorbent resin (B) having anaverage particle diameter of 400 μm and containing resin particles ofdiameters of less than 150 μm at a proportion of 5% by weight and havinga water content of 6% by weight.

REFERENTIAL EXAMPLE 3

[0120] In 5500 g of an aqueous solution of sodium acrylate having aratio of neutralization of 75 mol % (monomer concentration of 38%), 7 gof trimethylol propane triacrylate as the inner cross-linking agent wasdissolved. The resultant solution was deaerated with nitrogen gas andthen combined with 2.3 g of potassium persulfate and 0.11 g of1-ascorbic acid to set the monomer to polymerizing. At the time that thepolymerization was completed, the cross-linked hydrogel polymer producedin the meanwhile was finely divided and dried in a hot air drier at 150°C. The dried particles of the polymer consequently obtained werepulverized with a roll granulator and classified with an ASTM 20-meshsieve to obtain an irregular broken water-absorbent resin (C) having anaverage particle diameter of 380 μm and containing resin particles ofdiameters of less than 150 μm at a proportion of 5% by weight and havinga water content of 5% by weight.

REFERENTIAL EXAMPLE 4

[0121] An irregular broken water-absorbent resin (D) was obtained bymixing 100 parts of the water-absorbent resin (B) obtained inReferential Example 2 with a liquid composed of 0.5 part of glycerol asa first cross-linking agent, 0.05 part of ethylene glycol diglycidylether as a second cross-linking agent, 3 parts of water, and 0.75 partof isopropyl alcohol and heat-treating the resultant mixture at 200° C.for 35 minutes.

EXAMPLE 1

[0122] A water-absorbent agent (1) was obtained by mixing 100 parts ofthe water-absorbent resin (A) obtained in Referential Example 1 with aliquid composed of 0.05 part of ethylene glycol diglycidyl ether as across-linking agent, 0.5 part of lactic acid (pKa=3.66) as an additive,and 3 parts of water and heat-treating the resultant mixture at 120° C.for 40 minutes. The water-absorbent agent (1) was tested for properties.The results of the test are shown in Table 1.

EXAMPLE 2

[0123] A water-absorbent agent (2) was obtained by mixing 100 parts ofthe water-absorbent resin (A) obtained in Referential Example 1 with aliquid composed of 0.5 part of glycerol as a first cross-linking agent,0.05 part of ethylene glycol diglycidyl ether as a second cross-linkingagent, 0.5 part of citric acid (pKa=2.87) as an additive, 3 part ofwater, and 1 part of isopropyl alcohol and heat-treating the resultantmixture at 200° C. for 40 minutes. The water-absorbent agent (2) wastested for properties. The results of the test are shown in Table 1.

EXAMPLE 3

[0124] A water-absorbent agent (3) was obtained by repeating theprocedure of Example 2 while using 0.2 part of phosphoric acid(pKa=2.15) instead as an additive and tested for properties. The resultsof the test are shown in Table 1.

EXAMPLE 4

[0125] A water-absorbent agent (4) was obtained by mixing 100 parts ofthe water-absorbent resin (A) obtained in Referential Example 1 with aliquid composed of 1 part of ethylene carbonate as a first cross-linkingagent, 0.05 part of glycerol polyglycidyl ether as a secondcross-linking agent, 0.02 part of succinic acid (pKa=4.00) as anadditive, 2 parts of water, and 3 parts of isopropyl alcohol andheattreating the resultant mixture at 190° C. for 60 minutes. Thewater-absorbent agent (4) was tested for properties. The results of thetest are shown in Table 1.

EXAMPLE 5

[0126] A water-absorbent agent (5) was obtained by mixing 100 parts ofthe water-absorbent resin (B) obtained in Referential Example 2 with aliquid composed 0.5 part of glycerol as a first cross linking agent,0.05 part of ethylene glycol diglycidyl ether as a second cross-linkingagent, 3 parts of water, 0.75 part of isopropyl alcohol, and 0.5 part ofpolyaspartic acid (molecular weight 10000) as an additive andheat-treating the resultant mixture at 200° C. for 33 minutes. Thewater-absorbent agent (5) was also an absorbent composition (1) of thisinvention containing 0.5 part of polyaspartic acid (sodium salt).

EXAMPLE 6 AND EXAMPLE 7

[0127] The water-absorbent agents (6) and (7) were obtained by mixing100 parts of the water-absorbent resin (D) obtained in ReferentialExample 4 severally with an aqueous solution (10 parts of water) of 5parts of aspartic acid (molecular weight 50000) and an aqueous solutionof 1 part of polyglutamate (molecular weight 100000) as an additive anddrying the resultant mixtures at 120° C. These water-absorbent agents(6) and (7) obtained by adding the additive after adding a surfacecross-linking agent were also water-absorbent compositions (2) and (3)of this invention respectively containing polyaspartic acid (sodiumsalt) and polyglutamic acid (sodium salt).

EXAMPLE 8

[0128] A water absorbent composition (4) of this invention was obtainedby mixing 100 parts of the water-absorbent resin (3) obtained inReferential Example 3 with an aqueous liquid composed of 1 part ofpolysodium aspartate (molecular weight 10000) and 5 parts of water.

EXAMPLE 9

[0129] A water-absorbent resin composition (5) containing a polylysinesalt was obtained by dry blending 100 parts of the absorbent resin (4)obtained in Referential Example 4 with 10 parts of polylysinehydrochloride (molecular weight of 2000).

EXAMPLE 10

[0130] A water-absorbent composition (6) containing polysodium aspartatew as obtained by repeating the procedure of Referential Example 3 whilehaving 104.5 g of polysodium aspartate (molecular weight 10000)additionally dissolved in the aqueous sodium acrylate solution (5% assolids based on the monomer) thereby having the polysodium aspartateadditionally incorporated in the monomer being polymerized, thencontinuing the polymerization, and subjecting the resultant cross-linkedhydrogel polymer to fine division, drying, and pulverization. Thewater-absorbent composition (6) thus obtained, while showingsubstantially the same absorption capacity as the water absorbent resin(C) having used no polysodium aspartate in the polymerization, excelledthe water-absorbent resin (3) in the flowability after absorbingmoisture, the rustproofing effect, the gel stability, and the ability toresist static charging. By the pH titration, this composition (6) wasfound to have a watersoluble component of less than about 4% by weightcompared to the water-absobent resin (C).

[0131] Control 1

[0132] A water-absorbent agent (1) for comparison was obtained byfollowing the procedure of Example 1 while omitting the use of lacticacid as an additive. The water absorbent agent (1) for comparison wastested for properties. The results of this test are shown in Table 1.

[0133] Control 2

[0134] A water-absorbent agent (2) for comparison was obtained byfollowing the procedure of Example 1 while using 0.5 part of sodiumlactate in the place of lactic acid as an additive. The water-absorbentagent (2) for comparison was tested for properties. The results of thistest are shown in Table 1.

[0135] Control 3

[0136] A water-absorbent agent (3) for comparison was obtained byfollowing the procedure of Example 2 while using 0.5 part of sodiumcitrate in the place of citric acid as an additive. The water-absorbentagent (3) for comparison was tested for properties. The results of thistest are shown in Table 1.

[0137] Control 4

[0138] A water-absorbent agent (4) for comparison was obtained byfollowing the procedure of Example 2 while using 0.5 part of monosodiumsuccinate in the place of citric acid as an additive. Thewater-absorbent agent (4) for comparison was tested for properties. Theresults of this test are shown in Table 1.

[0139] Control 5

[0140] The water-absorbent resin (D) obtained in Referntioal Example 4was used in its unmodified form as a water-absorbent agent (5) forcomparison.

EXAMPLE 11

[0141] The water-absorbent compositions (1) through (5) conforming tothis invention and the water-absorbent resins (B) through (D) aswater-absorbent compositions (1) through (3) for comparison were testedfor absorption capacity without load to physiological saline solution,suction power to physiological saline solution, suction power to bovineblood, flowability after absorbing moisture, rustproofing effect, andability to resist static charging. The results of the test are shown inTable 2. It is clearly noted from the test results that thewater-absorbent composition of this invention excels also in theflowability after absorbing moisture, the rustproofing effect, and theability to resist static charging.

EXAMPLE 12

[0142] A water-absorbent structure (1) of this invention having adensity of 0.15 g/cc and a basis weight of 0.05 g/square centimeter wasobtained by dry mixing 100 parts of the water-absorbent composition (4)of this invention with 100 parts of ground pulp, pneumatically moldingthe resultant mixture in the form of sheet, and compressing the sheet.

EXAMPLE 13

[0143] Water-absorbent structures (2) through (5) of this invention andwater-absorbent structures (1) through (3) for comparison having adensity in the approximate range of from 0.13 to 0.17 g/cc and a basisweight in the approximate range of from 0.047 to 0.053 g/cm² wereobtained by following the procedure of Example 12 while using theabsorbent compositions (1) through (3) and (5) and the absorbentcompositions (1) through (3) for comparison in the plae of thewater-absorbent composition (4).

EXAMPLE 14

[0144] Water-absorbent structures of 5 cm in diameter were each punchedout of the water-absorbent structures (1) through (5) obtained inExamples 12 and 13 and the water-absorbent structures (1) through (3) tobe used as test pieces. When 10 cc of bovine blood was dropped onto thetest pieces, the water-absorbent structures of this invention showedbetter results in terms of the absorption speed than the water-absorbentstructures for comparison. It is clearly noted from the test resultsthat the water-absorbent structures containing the water-absorbentcompositions of this invention exhibit excellent suction power to bovineblood. Thus, the water-absorbent structures of this invention promise toprovide water-absorbent articles of heretofore unattainable quality.

EXAMPLE 15

[0145] A water-absorbent structure (6) of this invention having adensity of 0.18 g/cc and a basis weight of 0.05 g/cm² was obtained bydry mixing 75 parts of the water-absorbent composition (4) of thisinvention with 25 parts of ground pulp and 10 parts of thermoplasticfibers (ES filters 1.5 d, 10 mm, produced by Chisso Corporation),pneumatically molding the resultant mixture in the form of sheet, andthermally compressing the sheet.

EXAMPLE 16

[0146] Water-absorbent structures (7) and (8) of this invention andwater-absorbent structures (4) through (6) for comparison substantiallyequalling in density and basis weight were obtained by following theprocedure of Example 15 while using, in the place of the water-absorbentcomposition (4) of this invention of Example 15, water-absorbentcompositions differing in the absorption capacity without load tophysiological saline solution, the suction power to physiological salinesolution, and the suction power to bovine blood.

EXAMPLE 17

[0147] Water-absorbent articles (1) through (3) of this invention andwater-absorbent articles (1) through (3) for comparison were fabricatedby the following procedure using the water-absorbent structures (6)through (8) of this invention and the water-absorbent structures (4)through (6) for comparison obtained in Examples 15 and 16. A disposablediaper comprising a top sheet of polypropylene pervious to liquid, twosheets of tissue paper, a sample water-absorbent structure (weight about24 g) cut in a size of 12 cm×40 cm, a back sheet of polyethyleneimpervious to liquid including leg gathers and a waist gather, and twotape fasteners was manually assembled by joining the individualcomponent parts. The total weight of the diaper was about 45 g.

EXAMPLE 18

[0148] Ten babies were made to wear randomly 30 pieces each of thewater-absorbent articles (1) through (3) of this invention and thewater-absorbent articles (1) through (3) for comparison obtained inExample 17. The samples thus used on the babies were recovered andvisually examined to determine the ratio of leakage. The results areshown in Table 2. It is clearly noted from the test results that thewater-absorbent structures containing water-absorbent compositions at arelatively high concentration showed no satisfactory quality when thesuction power to bovine blood were less than 7 g/g even when the suctionpower to physiological saline solution were not less than 14 g/g. Thus,this invention provides means to select water-absorbent compositionswhich are effective in improving the qualities of water-absorbentarticles containing water-absorbant compositions at a highconcentration.

EXAMPLE 19

[0149] A water-absorbent structure (9) of this invention and awater-absorbent structure (7) for comparison substantially equalling indensity and basis weight were obtained by following the procedure ofExample 15 while using the absorbent agent (2) and the water-absorbentagent (1) for comparison respectively obtained in Example 2 and Control1 from the same water-absorbent resin (A) in the place of thewater-absorbent composition of this invention.

[0150] Disposable diapers were assembled with the water-absorbentstructure (9) and the water-absorbent structure (7) for comparison byfollowing the procedure of Example 17, to obtain a water-absorbentarticle (4) of this invention and a water-absorbent article (4) forcomparison. The samples of the water-absorbent article (4) and thewater-absorbent article (4) for comparison were tried on babies in thesame manner as in Example 18. In the test, the water-absorbent article(4) using the water-absorbent agent (2) obtained by the method ofproduction of this invention was found to show a ratio of leakage of3.3%, whereas the water-absorbent article (4) for comparison using thewater-absorbent agent (1) for comparison was found to show a ratio ofleakage of 13.3%. Thus, the water-absorbent agent obtained by the methodof production of this invention shows no detectable residue of across-linking agent used therein and exhibits a high absorption capacityunder load as without load and, therefore, is ideally adapted forwater-absorbent structures and water-absorbent articles such asdisposable diapers. TABLE 1 Amount of Absorp- residue of tion cap-surface Suction acity to Absorp- cross- power to phyolog- tion linkingphysiolog- ical saline capacity agent on ical saline without under loadsurface solution load (g/g) (ml/g) (ppm) (g/g) water-absorbent agent (1)51 31 ND 17.3 water-absorbent agent (2) 53 36 ND 17.5 water-absorbentagent (3) 52 35  2 17.0 water-absorbent agent (4) 52 35 ND 17.2water-absorbent agent (5) 52 33  2 17.4 water-absorbent agent (6) 51 29 5 17.0 water-absorbent agent (7) 50 28  5 17.3 water-absorbent agent 5329 30 16.9 (1) for comparison water-absorbent agent 52 30 12 17.0 (2)for comparison water-absorbent agent 52 34 12 17.1 (3) for comparisonwater-absorbent agent 53 33  9 16.9 (4) for comparison water-absorbentagent 51 29 15 17.5 (5) for comparison

[0151] TABLE 2 Absorption capacity to physiological Suction powerSuction Flowability saline solution to physiological power to afterRust- Ability to without load saline solution bovine blood absorbingproofing Gel resist static (g/g) (g/g) (g/g) moisture effect Stabilitycharging water-absorbent composition 52 17.4 10.3 good no rust noalmost, no (1) detrioration adhesion water-absorbent composition 50 17.07.2 good no rust no almost, no (2) detrioration adhesion water-absorbentcomposition 51 17.3 6.3 good no rust no almost, no (3) detriorationadhesion water-absorbent composition 40 15.3 8.4 good no rust no almost,no (4) detrioration adhesion water-absorbent composition 49 16.8 6.9good no rust no almost, no (5) detrioration adhesion water-absorbentcomposition 58 9.5 1.8 aggregation rust detrioration large (1) forcomparison adhesion water-absorbent composition 40 15.2 3.5 aggregationrust little large (2) for comparison detrioration adhesionwater-absorbent composition 51 17.5 5.2 little rust little large (3) forcomparison aggregation detrioration adhesion

[0152] TABLE 3 Physical properties of water-absorbent compositionAbsorption Suction Physical capacity power to Suction property underload to physiolog- power of absorbent physiological ical saline tobovine article saline sol- solution blood Ratio of ution (g/g) (g/g)(g/g) leakage (%) Absorbent article 40 15.3 8.4 6.7 (1) Absorbentarticle 52 17.4 10.3  6.7 (2) Absorbent article 50 17.0 7.2 10.0 (3)Absorbent article 40 16.2 2.3 20.0 (1) for comparison Absorbent article42 14.9 2.1 13.3 (2) for comparison Absorbent article 58 16.8 3.3 16.7(3) for comparison

What is claimed is:
 1. A method for the production of a water-absorbentagent which comprises mixing a water-absorbent resin containing acarboxyl group with an additive soluble in the aqueous solution of atleast one member selected from the group consisting of inorganic acids,organic acids, and polyamino acids and a cross-linking agent capable ofreacting with said carboxyl group.
 2. A method according to claim 1,which further comprises mixing the water-absorbent resin with saidadditive and said cross-linking agent and subsequently subjecting theresultant mixture to a heat treatment at a temperature in the range offrom 100 to 230° C.
 3. A method according to claim 1, which furthercomprises preparatorily mixing said additive and said cross-linkingagent and subsequently adding the resultant mixture to the absorbentresin.
 4. A method according to claim 1, wherein said additive is atleast one member selected from the group consisting of saturated organiccarboxylic acids, saturated inorganic acids of elements of Group 3 inthe periodic table of elements, and poly(monoaminodicarboxylic acids).5. A method according to claim 1, wherein the water-absorbent resincontaining a carboxyl group is obtained by polymerizing a hydrophilicmonomer having acrylic acid and/or a salt thereof as a main componentthereof.
 6. A method according to claim 1, wherein the cross-linkingagent capable of reacting with the carboxylic group is an epoxycompound.
 7. A method according to claim 1, wherein the aciddissociation index (pKa value) of said additive is in the range of from2.0 to 4.0.
 8. A method according to claim 4, wherein the additive is asaturated organic carboxylic acid.
 9. A method according to claim 8,wherein said saturated organic carboxylic acid is at least one compoundselected from the group consisting of citric acid, succinic acid, andlactic acid.
 10. A method according to claim 1, wherein the mixture iseffected by the use of from 0.01 to 5 parts by weight of water based on100 parts by weight of said water-absorbent resin.
 11. A methodaccording to claim 1, wherein the water-absorbent resin comprisesirregular broken particles of an average particle diameter of from 200to 600 μm and includes not more than 10% by weight of a fraction of lessthan 150 μm in diameter.
 12. A method according to claim 1, wherein saidadditive is used at a ratio in the range of from 0.01 to 5 parts byweight to 100 parts by weight of said water-absorbent resin.
 13. Amethod according to claim 1, wherein said water-absorbent resin has awater content in the range of from 1 to 10%.
 14. A water-absorbent agentobtained by adding from 0.005 to 8 parts by weight of an epoxy compoundto 100 parts by weight of a water-absorbent resin containing carboxylgroup, exhibiting an absorption capacity without load of not less than45 (g/g) and an absorption capacity under a load of 20 g/cm² of not lessthan 30 (ml/g) and having a residue amount of an epoxy compound contentof not more than 2 ppm.
 15. A water-absorbent agent according to claim14, wherein a hydroxyl group-containing saturated organic acid iscontained at a concentration in the range of from 0.01 to 5% by weight(based on said absorbent resin).
 16. A water-absorbent agent accordingto claim 14, wherein the absorption ratio under a load of 20 g/cm² isnot less than 35 (ml/g) and the epoxy compound is no longer detected.17. A water-absorbent composition comprising a polyamino acid (salt) anda water-absorbent resin containing a carboxyl group.
 18. A compositionaccording to claim 17, wherein said water-absorbent resin is across-linked polymer of acrylic acid.
 19. A composition according toclaim 17, wherein the ratio of said polyamino acid (salt) is from 0.01to 30 parts by weight to 100 parts by weight of said water-absorbentresin.
 20. A composition according to claim 19, wherein said polyaminoacid (salt) is at least one member selected from the group consisting ofpolyaspartic acid (salt), polyglutamic acid (salt), and polylysine(salt).
 21. A water-absorbent composition comprising an absorbent resinexhibiting an absorption capacity without load of at least 25 g/g tophysiological saline solution and a suction power of not less than 14g/g to physiological saline solution and a suction power of at least 7g/g to bovine blood.
 22. A water-absorbent structure comprising from 10to 95 parts by weight of a water-absorbent composition containing apolyamino acid(salt) and a water-absorbent resin having a carboxyl groupand from 90 to 5 parts by weight of hydrophilic fibers.
 23. Awater-absorbent structure according to claim 22, which further comprisesup to 40 parts by weight of thermoplastic fibers based on 100 parts byweight of said hydrophilic fibers.
 24. A water-absorbent structureaccording to claim 22, wherein the density is in the range of from 0.06to 0.5 g/cc.
 25. A water-absorbent structure comprising 50 to 95 partsby weight of a water-absorbent composition formed of a water-absorbentresin exhibiting an absorption capacity without load of at least 25 g/gto physiological saline solution and a suction power of not less than 14g/g to physiological saline solution and suction power of at least 7 g/gto bovine blood and 50 to 5 parts by weight of a hydrophilic fibers. 26.A water-absorbent structure according to claim 25, which furthercomprises up to 40 parts by weight of thermoplastic fibers based on 100parts by weight of said hydrophilic fibers.
 27. A water-absorbentstructure according to claim 25, wherein the density is in the range offrom 0.06 to 0.5 g/cc.
 28. A water-absorbent article furnished with thewater-absorbent structure set forth in any of claims 22 through 27and/or the water-absorbent composition set forth in any of claims 17 to21.